For migrating aircraft navigation and the independent position determination of aircraft from the traditional navigational aids (NAVAIDS), such as VHF Omni-direction Range (VOR) and Distance Measuring Equipment (DME), Air Navigation Service Providers (ANSPs) are moving toward performance based navigation (PBN) that is largely based and dependent on GPS to provide highly accurate positioning capability to aircraft. The high positioning accuracy of a GPS navigation-based system makes possible more efficient flight procedures such as the Area Navigation (RNAV) as well as Required Navigation Performance (RNP) procedures to be designed and implemented in the air space. Such new procedures make more efficient use of limited airspace, thereby improving the operational efficiency and capacity of the airspace over the existing air transportation systems. The use of GPS as a navigation source is also used by the automatic dependent surveillance-broadcast (ADS-B) system, which broadcasts aircraft position information from ADS-B equipped aircraft that is received by aircraft near the transmitting ADS-B equipped aircraft and ground air traffic control (ATC) stations. With ADS-B providing surveillance information to ATC, traditional Secondary Surveillance Radar (SSR) is no longer necessary as a primary surveillance system and is only used as a back-up surveillance means to the GPS-based ADS-B system.
However, reliance on GPS for providing both the navigation and surveillance services to the airspace is being recognized as having a significant risk due to the vulnerable nature of GPS signals. Specifically, the signals transmitted by GPS satellites are very low in power, so GPS receivers can be easily interfered or jammed by intentional or non-intentional radio frequency interference (RFI) sources, which causes the GPS receivers to stop functioning correctly. For example, an RFI source capable of interfering with/jamming GPS signals is available and can be purchased via internet merchandising at low cost, making it a real threat to the integrity of future aircraft navigation and surveillance systems.
Due to the vulnerability of GPS and the dependence of air navigation and surveillance system on GPS, an alternative means of navigation that is not dependent on GPS signals is needed.
Existing means of alternative navigation include DME/DME navigation, in which aircraft interrogates simultaneously multiple ground DME transponders and measures the ranges to the transponders. The position of the aircraft is then calculated using range-range positioning algorithms (as illustrated in FIG. 2). This method is a suitable alternative navigation system for aircraft equipped with an DME multi-channel scanner system but is not an economically feasible solution for non-commercial general aviation aircraft users. Additionally, because of the limited ranging accuracy provided by the DME system and limited coverage of existing ground DME transponder network, DME/DME is mostly suitable for RNAV-2 nautical miles (NM) operations, where the 2 NM means a horizontal position Total System Error does not exceed 2 NM 95% of the time. While RNAV-2 NM is sufficient for en-route operations, it is not sufficient for terminal operations where RNAV-1 NM or better is usually required. For some terminal operations. RNP capability is further required. To achieve RNP performance, redundant measurements are generally required for computing the integrity of the solution. The need for integrity poses higher demands on the DME infrastructure as well as DME/DME avionics. Because of this, it is not common to have DME/DME RNP better than 1 NM. Therefore, a more capable means of alternative navigation other than DME/DME is still needed.
Existing certified means to provide position information to ADS-B avionics are based on GPS or Wide-area Augmentation System (WAAS) due to highly stringent accuracy and integrity requirements of ADS-B. Since WAAS is part of the Global Navigation Satellite System (GNSS) and is also vulnerable to jamming and spoofing like GPS, and DME/DME does not meet the stringent accuracy and integrity requirements, a backup position source able to meet ADS-B requirements is still needed.
In addition, signal spoofing or fake signal broadcasting on the GPS frequencies as well as ADS-B frequencies can seriously impact the safety and efficiency of aircraft operations in the national airspace system (NAS). Therefore, what is needed is a more resilient alternative means of positioning and navigation as well as means to validate navigational and positional information, which is vital to aircraft operations in the NAS when GPS is unavailable or is spoofed.